Recently, the number of thefts of automobiles, and the number of thefts from automobiles are increased. Therefore, a theft alarm device for vehicle has spread, which issues alarm sounds if a suspicious person intrudes into a parked automobile. Such a theft alarm device for vehicle is equipped with an intrusion detection device that detects intrusion (presence) of a suspicious person (a human) into a monitored space (an interior of an automobile). This type of intrusion detection device is configured to emit ultrasonic waves with a specified frequency to the interior, and is configured to detect the frequency shift of reflected waves generated, resulting from the Doppler effect, associated with a movement of a human that is present in the interior (e.g., see Japanese Patent Application Publication No. 2007-183828).
Here, there is a need that the above-mentioned intrusion detection device is operated during parking of the automobile (a state where an engine has been cut). Then, because the intrusion detection device receives power supply from a battery which the automobile is equipped with, it is required that the intrusion detection device is capable of being operated with low power consumption, in order to prevent exhaustion of the battery. As a result, conventionally, the low power consumption has been attained by intermittently performing the transmission and reception of the ultrasonic waves.
However, in the state where the transmission and reception of the ultrasonic waves are performed intermittently as described above, there is fear that the detection accuracy is decreased if the intermittent time is lengthened for reducing the power consumption. Further, there is fear that the ultrasonic waves are reflected resulting from a movement of an object (e.g., falling of papers, a sunshade, or the like that is leaned against the interior wall) other than a human that is present in the interior, and accordingly the conventional intrusion detection device erroneously detects, as an intruder (human), the moving object other than the human.